1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) signal receiver, and more particularly, to an OFDM signal receiving apparatus and method of estimating a common phase error (CPE) of received OFDM signals using data subcarriers in addition to pilot subcarriers.
2. Description of the Related Art
A multicarrier based OFDM signal may be used in a DVB-T (Terrestrial Digital Video Broadcasting) system. DVB-T (Digital Video Broadcasting) is a pan-European broadcasting standard (ETS 300 744) for digital terrestrial television. DVB-T is directly compatible with MPEG2-coded TV-signals. The introduction of this digital service is already in progress in various European countries.
In OFDM systems, modulation and demodulation can be done digitally by computationally efficient Fast Fourier Transforms (FFT) of finite length, N. The orthogonality of the consecutive OFDM symbols is maintained by appending a cyclic prefix (CP) of length GI at the start of each symbol. The CP is obtained by taking the last v samples of each symbol and consequently the total length of the transmitted OFDM symbols is N+v samples. The duration of the FFT window N is the duration of the “useful period” ignoring the Guard Interval (GI) period during which the receiving antenna is presumably polluted by a mixture of the new symbol & the delayed versions of the previous one (i.e.: the echoes—the ghosts). The receiver discards the CP and takes only the last N samples of each OFDM symbol for demodulation by the receiver FFT.
The DVB-T standard determines FFT-length (N) of 2 k and 8 k. Thus, an OFDM-symbol consists of 2 k or 8 k sub-carriers respectively. However, not all of the sub-carriers can be used for data transmission. A number of the sub-carriers are used either for the spectral limitation of the transmission signal or for the transmission of pilot information.
A number of OFDM symbols are combined to form an OFDM DVB-T frame. One frame of an OFDM DVB-T Signal is composed of 68 symbols each having 1705 active carriers in a N=2K mode, or 6817 active carriers in a N=8K mode, respectively. The active carriers of each symbol include data subcarriers and pilot subcarriers. The data subcarriers are digital signals corresponding to audio/video information to be transmitted and received and the pilot subcarriers are digital signals to be used for synchronization, mode detection, channel estimation, etc. A pilot subcarrier is inserted between neighboring data subcarriers in a predetermined position.
Orthogonal Frequency Division Multiplex (OFDM) systems are very sensitive to phase noise (e.g., caused by oscillator instabilities). The phase noise may be resolved into two components, namely the Common Phase Error (CPE), also known as average phase noise offset, which affects all the subchannels equally, and the Inter Carrier Interference (ICI), which is caused by the loss of orthogonality of the subcarriers.
FIG. 1 is a block diagram of a conventional OFDM signal receiver 100. Referring to FIG. 1, the OFDM signal receiver includes an RF (Radio Frequency) module 110, a demodulator 120, a frequency synchronization (FS) unit 130, a FFT (Fast Fourier Transform) unit 140, an equalizer (EQ) 150, a Common Phase Error (CPE) estimation and correction unit 160, and a demapper 170.
The demodulator 120 demodulates a digital OFDM signal output from the RF module 110 (received in a signaling format such as QPSK, BPSK or QAM), to generate an in-phase (I) signal (referred to as I-signal hereinafter) and a quadrature-phase (Q) signal (referred to as Q-signal hereinafter), which are complex signals. The demodulator 120 down-converts the digital OFDM signal is into a low-frequency signal and demodulates it. A frequency offset of the demodulated signal is compensated while the demodulated signal passes through the frequency synchronization (FS) unit 130. The frequency synchronization (FS) unit 130 estimates the frequency offset from the demodulated signal. When an estimation error is generated due to noise and channel distortion, the signal compensated by the frequency synchronization unit 130 may include a residual frequency offset. The signal compensated by the frequency synchronization unit 130 passes through the FFT unit 140, and is then equalized by the equalizer (EQ) 150. The CPE estimation and correction unit 160 estimates and corrects a Common Phase Error (CPE) equally generated in all subcarriers of the OFDM signal. A CPE is the difference between the phase of the original (transmitted) signal and the phase of a received signal, and is equally generated in all subcarriers. It is known that the CPE may be caused by a residual frequency offset and phase noise in the output of an oscillator included in the RF module 110. In the aforementioned conventional technique, pilot subcarriers are used to estimate the CPE. The pilot subcarriers may be used to transmit promised (predetermined, expected) values between a transmitter and a receiver in an OFDM system. The pilot subcarriers may be used by the receiver to estimate a frequency offset or channel distortion.
In general, the CPE can be estimated using phase rotation generated in the pilot subcarriers because it is a common phase error generated in all subcarriers. The CPE may equal a value, Δ{circumflex over (φ)}r, obtained by estimating the quantity of phase rotation generated in carriers due to a residual frequency offset and can be represented as follows:
                                          Δ            ⁢                                                  ⁢                                          ϕ                ^                            r                                =                                    tan                              -                1                                      ⁡                          [                                                ∑                                      k                    ∈                    P                                                  ⁢                                                      R                    k                                    ·                                      S                    k                    *                                                              ]                                      ,                  P          =                      {                                          -                21                            ,                              -                7                            ,                              +                7                            ,                              +                21                                      }                                              [                  Equation          ⁢                                          ⁢          1                ]            wherein k represents a subcarrier index and Sk and Rk respectively denote a transmitted (expected) value and a received value with respect to the pilot subcarriers.
The CPE estimation and correction unit 160 extracts pilot subcarriers from the equalized signal output from the equalizer 150, multiplies complex numbers of the extracted pilot subcarriers Rk by conjugate complex numbers of the transmitted original (expected) pilot subcarriers Sk, sums up the multiplication results, and estimates the tan−1 value of the complex value obtained from the overall result to be the quantity of phase rotation, Δ{circumflex over (φ)}r. In Equation 1, the set P is an example of an IEEE802.11a WLAN (Wireless Local Area Network) standard and, in such a case, subcarriers −21, −7, +7 and +21 (of 64 subcarriers −32 through +31) are used as pilot subcarriers.
As described above, the CPE can be estimated using a phase variation between the transmitted (expected) pilot value and the received pilot value. However, a CPE estimation error can be generated when there is noise or channel distortion. Although the number of pilots can be increased to improve CPE estimation accuracy, the total transmission rate of the system would be reduced. Thus, the number of pilots should be appropriately determined. In particular, when a total of four pilots are used, as described above, conventional CPE estimation accuracy is low and thus the system can become sensitive to noise and channel distortion.